1. Field of the Invention
The present invention relates to a resin sealed semiconductor device and it is primarily useful in a power transistor.
2. Description of the Prior Art
In a resin sealed, relatively small size power transistor is disclosed in U.S. Pat. No. 3,478,420, a semiconductor pellet is fixed on a copper header, electrodes of the semiconductor pellet are connected to corresponding lead directly or through wires and the semiconductor pellet and the lead wires are surrounded and sealed by a resin mold such as silicon resin, epoxy resin, etc. on the header.
In such a resin sealed semiconductor device, because of a large difference between coefficients of thermal expansion of the header and the resin mold, the resin mold may peel-off the copper header or the resin mold may be cracked after the completion of the resin sealing or during a heat cycle test, a kind of life expectancy test.
Accordingly, in the small size power transistor, a projection is formed at a portion on the side of the header covered with the resin mold so that the projection constrains the shrinkage of the resin mold toward the center thereof immediately after the completion of the resin sealing for preventing the peel-off of the resin mold and the cracking of the resin mold which would otherwise occur by the difference between the coefficients of thermal expansion of the header and the resin mold (header: 17 .times. 10.sup.-6 deg.sup.-1, resin mold: 21 - 22 .times. 10.sup.-6 deg.sup.-1).
In a relatively large power transistor in which a flange having a heat sink is used as a support for the semiconductor pellet, however, the peel-off of the resin mold and the cracking of the resin mold due to the difference between the coefficients of thermal expansion may be prevented to a certain degree by providing the projection at a portion of the flange like in the small size power transistor but cracking may occur at a thin portion of the resin mold at which the projection is formed and the contact between the resin mold and the flange is so insufficient that the resin mold may be peeled off. Particularly in a semiconductor device in which the semiconductor device is clamped to a circuit board or the like by clamping the flange on both sides thereof, it is difficult to form the projection on that side of the flange on which a mounting bore is formed and a gap may be created between the resin mold and the flange by the clamp. This poses a problem of anti-humidity property of the resin mold.
Although the occurrence of the cracking of the resin mold may be relieved by coating the projection with a thick resin body, the use of increased amount of resin mold leads to the increase in cost and hence the problem of peel-off of the resin mold still remains unresolved.